(ABC News) – The coral on the sea floor around the Pacific island of Kiritimati looked like a boneyard in November — stark, white and lifeless. But there was still some hope.

This month, color returned with fuzzy reds and browns, but that's not good news. Algae has overtaken the lifeless coral on what had been some of the most pristine coral reefs on the planet, said University of Victoria coral reef scientist Julia Baum after dozens of dives in the past week. Maybe 5 percent will survive, she estimated.

"What it really looks like is a ghost town," Baum said. "It's as if the buildings are standing but no one's home."

Kiritimati is where El Niño, along with global warming, has done the most damage to corals in the past two years, experts said. While dramatic images of unprecedented total bleaching on Australia's Great Barrier Reef are stunning the world, thousands of miles to the east conditions are somehow even worse.

"This El Niño has its most powerful grip right at this spot," said Georgia Tech climate scientist Kim Cobb in a telephone interview from the island 2,000 miles south of Hawaii.

About 36 percent of the world's coral reefs — 72 percent of the U.S. reefs — are in such warm water they are under official death watch, and that could rise to up to 60 percent of the world's coral by July, said Mark Eakin, the coral reef watch coordinator for the U.S. National Oceanic and Atmospheric Administration. Eakin said Kiritimati was the worst he's seen, with America Samoa a close second.

"It is unprecedented," Baum said. "This is absolutely the most intense response, the most dramatic death of a coral reef from an El Niño event."

The island has been on the highest level of alert for coral stress since June 2015. NOAA's Eakin wasn't part of the Baum-Cobb team, but when he saw their photos, he emailed Baum: "I was simply aghast at the pictures. I expected the worst, but still wasn't prepared for those."

He called it "gruesome."

It's the heat that's killing the coral. In December, temperatures at Kiritimati peaked at 88.5 degrees (31.4 degrees Celsius) and have been about 5 to 7 degrees warmer than normal. That's the kind of temperature spike that can be the difference between life and death for coral, Eakin said. [more]